Well fracturing



United States Patent 3,104,706 WELL FRACTURENG Louis H. Eilers andArthur Park, Tulsa, Okla., assignors to The Dow Chemical Company,Midland, Mich, a corporation of Delaware N0 Drawing. Filed Oct. 15,1959, Ser. No. 846,562 12 Claims. (Cl. 166-42) The invention pertains toincreasing the flow of a fluid from a fluid-bearing subterraneanformation penetrated by a well by the use of an explosive mixture.

Fluids, e.g., oil or gas, contained in the subterranean formation, areusually obtained by drilling a well into the formation and providing apressure differential to force the fluids from the formation to theearths surface.

Fluid is contained in spaces between the solid portions of theformation, such spaces varying greatly in size and shape and are usuallydescribed a pores, cavities, channels, and the like. The ratio by volumeof such spaces to the volume of the formation is known as porosity. Theextent to which the spaces, e.g., pores, are interconnected and therebyprovide passage of fluids to the formation is known as permeability.

The flow of a fluid from a formation by providing a pressuredifferential is often greatly hampered and sometimes completely stoppedbecause thefluid flow from appreciable, and sometimes extensive,portions of the formation is obstructed by more or less impermeableintervening rock or other compact solids, thus blocking free flow of thefluid to' the well.

Various methods of stimulating the flow of fluids from wells penetratingfluid-bearing formations which are at low levels of production includesuch methods as acidizing, hydraulic fracturing, and detonating origniting an explosive composition which has been positioned in orinjected down the well.

Acidizing consists essentially of injecting an aqueous solution of anacid, usually hydrochloric acid, often containing an inhibitor to metalattack, into a well requiring reatment. The acid reacts with theformation and causes cavities and channels to be created in theformation. Hydraulic fracturing generally refers to injecting a liquid,often a thickened oil or an oil-water emulsion, and preferablycontaining suspended par-ticulated solids as a propping agent, e.g.,sand, into a formation to effect fracturing of the formation. Acidizingand hydraulic fracturing may be combined by employing an acidicfracturing fluid.

Stimulating the flow of a fluid from a formation by the use of anexplosive includes what is generally referred to as shooting the wellwith such an explosive as nitroglycerine, which has long been employedfor such purposes.

Acidizing is inherently limited to formations composed substantially ofa carbonate-containing rock, e.g., limestone or dolomite. Hydraulicfracturing requires elaborate pumps, and blending equipment when sand isemployed, to provide the high pumping rates required for effectivehydraulic fracturing. Furthermore, hydraulic fracturing is generallydiflicult to direct into the portions of the formation where improvedcommunication is most desired.

The use of liquid explosives such as nitroglycerine is BifidfldbPatented Sept. 24, 1963 ice fraught with the dangers associated withhandling them. Solid explosives such as nitrocellulose and dynamite aredetonated, in the wellbore itself and, therefore, are definitely lessdesirable than an explosive composition which could be forced back intothe formation prior to detonation.

Fracturing a subterranean formation penetrated by a Well by injectingdown the well and into the formation a mixture of liquids andsubsequently detonating the mix ture is described in copendingapplication S.N. 838,051, filed September 4, 1959, now Patent No.3,075,463. The liquid mixture first injected down the Well according tothat application consists of an oxidizing agent, e.g., fuming nitricacid, and a reducing agent, e.g., .a nitroalkane such asZ-nitro-propane, the reducing agent usually being referred to therein'as a fuel. The mixture of oxidizing and reducing agents is subsequentlydetonated either by a shock or heat type igniter e.g., heat produced bypassing an electric current through a conductor in contact with saidemulsion or by injecting into the well and into contact with at least aportion of the mixture already injected into the well a liquid igniterwhich chemically ignites the mixture in the formation. Chemicaldetonation or ignition of the liquid mixture thus injected into theformation is the preferred way of practicing the invention. The igniteris usually aniline, furfuryl alcohol, or a mixture thereof. However, anumber of other effective liquid igniters are described in saidapplication.

Although the method of fracturing described in application S.N. 838,051represents a definite improvement in the fracturing of Wells, thereducing agent and oxidizing agent selected therein are limited to thosewhich do not immediately separate into phases upon admixture together.For example, the use of such fuels (reducing agents) as gas oils, dieselfuel, and kerosene are not fully satisfactory when employed with thepreferred oxidizing agent named therein, viz., fuming red nitric acid orfuming white nitric acid, due to their immiscibility with the ad A need,therefore, exists for an improved Well fracturing method employing aliquid mixture which is safe for necessary handling, which readilypenetrates a subterranean formation, which employs a readily availablefuel with a highly effective and a readily available oxidizing agent,which is a substantially stable one-phase liquid for a sufficient timeto inject it into a Well and back in the formation, and which isdetonatable upon being subjected to shock or heat or to contact with aliquid chemical igniter.

The invention consists of injecting down a well penetrating asubterranean formation an emulsion consisting of an oxidizing agent, areducing agent which hereinafter will usually be referred to as a fuel,and an emulsifying agent and subsequently detonating or igniting theemulsion thus injected by subjecting the emulsion to shock or heat, orby injecting down the well and into contact with at least a portion ofthe emulsion a liquid chemical which produces a hypergolic compositionwhen thus brought in contact with the emulsion. An explosion is thusproduced which propagates through the remaining emulsion, which haspermeated the formation, producing fractures in the formation.

A preferred embodiment of the invention includes the step ofincorporating into the emulsified oxidizing and 9 reducing agent mixturean activator selected from the class consisting of aliphatic esters ormixtures of such esters containing between 4 and 14 carbon atoms permolecule or nitric acid solutions of inorganic oxygen-containing saltsthat readily yield reactive oxygen when present in the hypergoliccomposition.

If, desired, the treatment of the well in accordance with the inventionmay be either preceded by or followed by conventional fracturing or, ifthe Well is already fractured, it may betreated according to theinvention and then, if desired, again fractured by hydraulic fracturing.It has been found that combining conventional fracturing with theimproved fracturing method of the invention has resulted in particularlyimproved production from wells so treated.

The reducing agent employed in the practice of the invention may be anyfuel which does not spontaneously react with the oxidizing agent and isemulsifiable therewith, but which can be ignited .or detonated by heat,shock, or chemical means. Illustrative of a reducing agent or fuel toemploy are nitroalkanes, nitroaranes, aliphatic alcohols and ethers,suspensions :of particulate hydrocarbon solids in a hydrocarbon liquid,alkyl dihalides, and liquid hydrocarbons. The preferred reducing agent,i.e., fuel, is a liquid hydrocarbon such as a petroleum fraction, e.g.,diesel oil, gas oil, distillate, naphtha, kerosene, or mixtures thereof.

Oxidizing agents useful in the practice of the invention may be anyoxidizing agent that does not spontaneously react with the fuel atconditions. found prior to injection into the well. Among such oxidizingagents are liquid halogens, hydrogen peroxide, perchloric acid, nitricacid, and fuming nitric acid. Nitric acid or fuming nitric acid may haveadmixed therewith concentrated sulfuric acid, or preferably fumingsulfuric acid. Fuming sulfuric acid thus employed usually containsbetween 15 and 20 percent SO dissolved in concentrated H 80 The preferred oxidizing agent to employ is fuming nitric acid, particularly redfuming nitric acid. Red fuming nitric acid consists of an aqueoussolution of HNO having dissolved therein N Red fuming nitric acid may beprepared by passing N0 into nitric acid, preferably concentrated nitricacid, or by reacting .an alkali metal nitrate, e.g., KNO with fuming H80 A discussion of fuming nitric acids and their preparation may befound in Mellors A Comprehensive Treatise of Inorganic and TheoreticalChemistry, vol. VIII, page 5 63 ot seq. Concentrated nitric acid usuallyis considered to be about 68 percent HNO dissolved in water and at thatpercentage has a stable boiling point. Nitric acid having aconcentration of HNO which is appreciably greater than 68 percent isknown as white fuming nitric acid. Such white fuming nitric acid usuallycontains between 85 and 98 percent HNO the balance consisting of waterand up to about 0.5 N0

The ratio of the oxidizing agent to the fuel is not highly critical butit is recommended that more than enough of the oxidizing agent bepresent to convert all the hydrocarbons in the fuel to C0, C0 and H 0.

The emulsifying agent employed in the practice of the invention may beany emulsifier which is capable of forming a stable emulsion between theoxidizer and the fuel. By stable emulsion is meant one that will notstratify appreciably within the time required to inject the emulsionthus made into the well and subsequently detonate the emulsion thusinjected. In some well treating operations to minutes might be anadequate time but it is recommended that an emulsifier be employed thatwill make an emulsion which is substantially stable for at least 30minutes and preferably for 60 minutes or longer. Preferred emulsifiersto employ in the practice of the in vention are gilsonite, asphalt, andtarry residues from coal tar distillations, e.g., quinoline residues.Asphaltum, more commonly referred to as asphalt, is a bituminoussemi-solid hydrocarbon mixture thought to have been formed by theevaporation of the more volatile consti-tuents therefrom. It is eitherblack or brown in color with a pitchy luster. It is found naturally,often in exposed beds, e.g., those of western United States andTrinidad. Gilsonite or uintaite is a variety of asphalt originallyobtained largely in Uinta County, Utah, having superior emulsifyingproperties for oil-acid mixtures. The amount of emulsifier to employ inthe practice of the invention is between 40 and 400 pounds per 1000gallons of emulsion. The preferred range of emulsifier to employ isbetween and 250 pounds per 1000 gallons of emulsion.

When an activator, e.g., an aliphatic ester or nitric acid solution ofan oxygen-yielding compound, is employed to facilitate the detonation orignition of the oxidizing and reducing agent mixture it is usuallyemployed in an amount of between about 5 and 20 percent by weight of theemulsion. Among such esters are lower alkyl acetates, butyrates, andlaurates. Among such oxygen-yielding compounds are potassium and sodiumchlorate, perchlorate, permanganate, and dichromate.

illustrative of the chemical igniter to employ in the practice of theinvention .are especially powerful oxidizing or reducing agents. Thelatter are more commonly employed among which are aniline, alkylsubstituted aniline, furfuryl alcohol, dichlorpropene,monochloropropane, ethylenediamine, diethylenetriamine, and alkylmercaptans. A particularly effective igniter liquid is a mixture offurfuryl alcohol and aniline.

The invention is preferably carried out by admixing an asphalt-typeemulsifier with a fuel such as kerosene, in an amount stated above, andthen admixing the oxidizing agent, preferably red fuming nitric acid,with the fuel containing the emulsifier, agitating the mixture thus madeto form an emulsion, and injecting the emulsion down the well and thenceinto the formation. If an ester is employed to aid the subsequentignition, it may be admixed either with the oxidizing agent or thereducing agent or in the mixture of the two and thereby form a part ofthe emulsion thus injected into the well.

The mixture so injected is then detonated. If a high velocity detonatoris employed, e.g., a blasting cap or Primacord(pentaerythritol-tetranitrate), such may be placed in the well eitherbefore or after the injection of the emulsified oxidizing and reducingagent mixture. If the emulsion is to be ignited by a chemical means,which is the preferred mode of practicing the invention, such chemicaligniter is injected down the well and into contact with the emulsion.

The following procedure is one mode of practicing the invention: Apredetermined quantity of the emulsion prepared as above is injecteddown the well by a pumping means, e.g., one similar to that used inacidizing, and a rubber plug is positioned in the Wellbore, convenientlynear the top thereof. A buifering liquid usually consisting of the fuelemployed in the preparation of the emulsion, is then pumped down thewell on top of the plug, the plug thereby forced downward until thebuffering liquid is all in the wellbore. A second plug is placed in thewellbore in a manner similar to the placement of the first plug. Thechemical igniter liquid then is pumped down the well on top of thesecond .plug thereby forcing the plug downward until the igniter liquidis in the wellbore, a third plug is then placed in the wellbore in amanner similar to the placement of the two previous plugs. Thereafter aflush liquid, usually more of the fuel previously employed, is pumpedinto the well on [top of the third plug with sufiicient force and insufficient quantity to force the plugs and the liquids therebelow andtherebetween out of position and down into the -Well whereby the igniterliquid is brought into contact with the explosive emulsion in the bottomof the well and in the adjacent formation, thereby rendering ithypergolic. The ensuing explosion shatters and fractures the formationthereby providing passageways therethrough for fluid from the formationand improving their production therefrom. The technique followed forinjecting the mixture of oxidizing and reducing agents into the well andsubsequently injecting the igniter liquid is largely a matter of thepractice preferred. The above mode is set out only as one which has beenfound to be particularly convenient and effective.

Examples were run to lilustrate the practice of the invention employingasphalt type emulsifiers with kerosene and red fuming nitric acid whichconsisted of 98 to 99.5 percent HNO (balance water) having dissolvedtherein about 14 percent by weight N The procedure was as follows: 0.12gram of the emulsifier set out in Table I was admixed with 1 milliliterof kerosene and the mixture thus made admixed with 4 milliliters of thered fuming nitric acid and emulsified by forcing the mixture thus madethrough a small aperture in accordance with known emulsifyingtechniques. The stability of the emulsions thus made was then observedby placing each of the thus made emulsions in a glass, verticallydisposed, graduated vessel and observing the length of time required forabout V2 the emulsion to break and the time required for substantiallyall the emulsion to break. The time required for breaking of theemulsion is set out in Table I.

TABLE I Emulsification of Kerosene and Red Filming Nitric Acid Volumekerosene-l ml. Volume red fuming nitric acid-3 ml. Weight emulsifier0.l2g.

1 Standard Gilsonite; American Gilsonite Col; softening point, 373 F.

2 Select Gilsonite; American Gilsonite 00.; softening point, 280 F.

3 Asphalt that has been treated by blowing oxygen gas therethrough.Berrys blown asphalt No. 566; Berry Asphalt 00.; melting point, 270 F.

An examination of the results of Table I shows that a stable emulsionwas prepared by adding the emulsifier to the kerosene and thereafteradding thereto the fuming nitric acid and emulsifying. The resultsfurther show that gilsonite is superior to blown asphalt or quinolineresidues as an emulsifier for use in the practice of the invention.

Emulsions were again prepared following the procedure employed for thoseset out in Table I except that the emulsifier was added to the fumingnitric acid instead of to the kerosene and the acid-emulsifier mixturethereafter admixed with the kerosene. Emulsions were thus formed whichwere satisfactory but less stable than those prepared in Table I whereinthe emulsifier was added to the kerosene instead of to the fuming nitricacid prior to emulsifying the acid and kerosene.

Emulsions were again prepared following the procedure employed in TableI, placed in a ventilated hood behind a suitable protective wall butwhich provided observation of the emulsions, and 0.1 milliliter offurfuryl alcohol added to each emulsion by means of a dropper. In allcases very rapid burning of the emulsions occurred thereby showing thehypergolic character of the mixture consisting of the emulsion and thechemical detonator or igniter, viz., furfuryl alcohol.

-To show the effect of adding a suitable activator to the emulsion ofoxidizing and reducing agents employed according to the invention, priorto detonation, examples were run according to the folowing procedure: To0.85 milliliter of kerosene was added, 0.1 gram of the standardgilsonite employed in the examples of Table I and 2.5 milliliters redfuming nitric acid. The mixture was emulsified by passing it through anaperture as in the preparation of the emulsion of the examples of TableI. Thereafter, 0.5 milliliter of one of the esters set out in Table IIwas added to each of three examples. One of the examples was run as ablank and therefore no ester was added thereto. The emulsion thusprepared containing the ester was placed in a ventilated hood behindsuitable protection where it could be observed and a measured quantityof furfuryl alcohol added thereto, dropwise, until ignition occurred.The ester employed, the amount of igniter used, and the length of timeof burning are set out in Table II.

TABLE II Efiects of Activators 0n Burning Rates Fuel-0.85 ml. keroseneOxidizer2.5 ml. red fuming nitric acid Emulsifier0.l g. standardgilsonite Igniterfuriuryl alcohol (added dropwise) Igniter BurningExample N 0. Activator added used in time in ml. min.

None. 0. 4 2. 0 0.5 ml. Ethyl acetate 0.3 1. 5 0.5 ml. Amyl acetate0.2 1. 3 0.5 ml. Ethyl laurate 0.2 1.3

Reference to Table II shows that the presence of the aliphatic estersdefinitely reduced the requirement for the amount of igniter and thetime required for the emulsion to burn completely.

The examples of Table II were repeated employing nitric acid solutionscontaining about 0.5 gram of potassium chlorate, patassium permanganate,or potassium di chromate. The amount of igniter required and rate ofburning were similarly alfected as when the ester was employed.

Since the burning rate of a combustible mixture is accelerated byincreased pressure and the force of an explosion of an explosive mixtureis also accelerated by increased pressure, the following examples wererun. The procedure employed was as follows: milliliters of 20 to 40.mesh sand (U.S. Standard Sieve Series) were placed in a bomb made of abull plug provided with a pressure gauge. The bomb was 4 inches indiameter and had a volume of 2000 milliliters. The sand contained 40milliliters of pore space, i.e., it had a porosity of 40 percent.

The bomb was provided at the top with a means for introducing nitrogengas and for venting the bomb and with an assembly fior introducingliquids consisting of three inch diameter pipe lines, two of whichmerged into an aluminum nozzle prior to entering the bomb, therebyproviding for the entrance of only two separate lines into the bomb.Each line had a flow control means therein. Each of the three lines ledfrom a separate supply vessel, each of which was provided with nitrogengas pressure. Into one supply vessel connected to one of the two merginglines was placed a solution consisting of 12 milliliters of kerosene,0.6 gram of standard gilsonite, and 0.6 milliliter of amyl acetate andinto the supply vessel connected to the other of the two merging lineswere placed 30 milliliters of fuming red nitn'c acid. Into the supplyvessel connected to the third line, which entered the bomb directly,were placed 2 milliliters of furfuryl alcohol.

Nitrogen gas pressure was applied to the two supply vessels, one ofwhich contained the organic solution of kerosene, ester and emulsifierand the other of which contained the fuming nitric acid, and the flowcontrol means in the lines were opened simultaneously. The contents ofthe two vessels were thus simultaneously forced through the nozzlethereby forming an emulsion of the thus merged acid and organic solutionas they entered the bomb. Flow through the lines thus employed was thenclosed, the flow control means in the third line opened, and thefurfuryl alcohol forced by nitrogen pressure into the bomb and intocontact with the emulsified fuming Efiect of Pressure on ExplosiveEmulsion I Pressure on Increase in systemin pressure on atmosignition inphcres psi.

Example No. Resulting reaction Igniter burned partially. lgniter burnedcompletely.

Explosion occurred.

The eiiect of increasing the pressure on the explosive emulsion inaccordance with the invention as shown in Table ll, is to intensify theexplosion which ensues upon detonation. Such intensified explosionemphasizes the enhancing effect of increased pressure on the ensuingreaction. A pressure of 34 atmospheres, it is to be observed, isdefinitely less uh-an the pressure usually encountered in oil and gasproducing formations. Therefore, at the greater pressures usuallyexisting in producing Zones, a much more pronounced explosion wouldresult.

A number of advantages can be realized in the practice of the invention,among which are all the advantages of US. application SN. 838,051 are tobe realized but in addition'thereto: lower grade and more readilyavailable fuel may be employed; emulsifying agents useful in thepractice of the invention are procurable at relatively low cost; the useof an activator of the type illustrated by the aliphatic esters reducesthe quantity of igniter liquid required and shortens the burning periodwhich is evidence of a more violent reaction and consequently a moreshattering and effective fracturing operation on a formation beingtreated.

Having described the invention, what is claimed and desired to beprotected by Letters Patent is:

1. A method of fracturing a subterranean formation penetrated by a wellcomprising injecting down the well and into the formation 'a potentiallyexplosive emulsion consisting essentially of a liquid reducing agentselected from the class consisting of nitroallcanes, nitroaranes, alkylalcohols, alkyl ethers, suspensions of particulate hydrocarbon solidsdispersed in a liquid hydrocarbon,

alkyl dihalides, and liquid hydrocarbons, (2) an oil-water emulsifyingagent, and (3) an oxygen-containing compound which releases oxygen forreaction with the reducing agent at and above the ignition temperatureof the reducing agent and subsequently detonatiug said emulsion in theformation.

&

2. The method of claim 1 wherein an excess of the oxidizing agent overthe stcichiometric amount required to react fully with the reducingagent is employed.

3. The method of claim 1 wherein detonation is efiected by means of ahigh velocity shock-type detonator.

4. The method of claim 1 wherein detonation is effected by passingelectrical current through a conductor in contact with said emulsion.

5. The method of claim 1 wherein detonation is effected by injectingdown a well and into contact with a portion of said emulsion a liquidchemical which renders said emulsion 'hypergolic.

6. The method of fracturing a subterranean formation penetrated by awell comprising injecting down the well and into the formation apotentially explosive emulsion consistirw of (l) a reducing agentselected from the class consisting of nitroallcanes, nitroaranes, alkylalcohols, alkyl ethers, and suspensions of particulate hydro-carbonsolids dispersed in a liquid hydrocarbon, and alkyl dihalides, andliquid hydrocarbons, (2) an emulsifying agent selected from the classconsisting of gilsonite and asphalt, and (3) an oxidizing agent selectedfrom the class consisting of nitric acid red fuming nitric acid, whitefuming nitric acid, mixtures of concentrated sulfuric acid and fumingnitric acid, mixtures of fuming sulfuric acid and fuming nitric acid,liquid halogens, hydrogen peroxide,

and perchloric acid, and deton-ating the emulsion thus injected into thewell.

7. The method of claim 6 wherein the reducing agent is a liquidhydrocarbon.

8. The method of claim 6 wherein detonation is effected by means of apentaerythritoltetranitrate cord.

9. The method of claim 6 whereby detonation is ef- 'fected by injectingdown the well and into contact with at least a portion of said emulsiona liquid chemical igniter selected from the class consisting of aniline,alkyl substituted aniline, furfuryl alcohol, ethylene diamine, butylamine, diethylene amine, dichloropropene, monochloropropane, andmixtures thereof.

10. The method of claim 9 wherein the liquid detonatoris a mixture offurfuryl alcohol and aniline.

11. The method of claim 6 wherein the emulsion contains between 5 and 20percent of an activator selected from the class consisting of aliphaticesters containing between 4 and l4 carbon atoms per molecule andmixtures thereof.

12. The method of claim 11 wherein detonation is eifected by a liquidchemical igniter.

References Cited in the file of this patent UNITED STATES PATENTS

1. A METHOD OF FRACTURING A SUBTERRANEAN FORMATION PENETRATED BY A WELL COMPRISING INJECTING DOWN THE WELL AND INTO THE FORMATION A POTENTIAL EXPLOSIVE EMULSION CONSISTING ESSENTIALLY OF A LIQUID REDUCING AGENT SELECTED FROM THE CLASS CONSISTING OF NITROALKANES, NITROARANES, ALKYL ALCOHOLS, ALKYL ETHERS, SUSPENSIONS OF PARTICULATE HYDROCARBON SOLIDS DISPERSED IN A LIQUID HYDROCARBON, ALKYL DIHALIDES, AND LIQUID HYDROCARBONS, (2) AN OIL-WATER EMULSIFYING AGENT, AND (3) AN OXYGEN-CONTAINING COMPOUND WHICH RELEASES OXYGEN FOR REACTION WITH THE REDUCING AGENT AT AND ABOVE THE IGNITION TEMPERATURE OF THE REDUCING AGENT AND SUBSEQUENTLY DETONATING SAID EMULSION IN THE FORMATION. 